Benzene is considered to be environmentally hazardous. As a result, the State of California and the United States Environmental Protection Agency have instituted regulations to limit the amount of benzene which may be present in gasoline. As of January 2011, the US MSAT-2 (Mobile Source Air Toxics) regulation requires reduction of this annual average benzene content in gasoline to no greater than 0.62 volume %.
One known route for reducing the benzene content of gasoline is to selectively alkylate the benzene using a lower olefin. For example, Holtermann et al U.S. Pat. No. 5,149,894 describes a process for converting benzene to alkylated benzenes in a gasoline blend stock. The process involves contacting a benzene-containing gasoline blend stock with a C2 to C4 olefin stream in the presence of a catalyst containing the zeolite, SSZ-25, to produce an alkylated light hydrocarbon stream with reduced benzene content.
Cheng et al. U.S. Pat. No. 5,545,788 describes a process for the production of a more environmentally suitable gasoline by removing a substantial portion of benzene in gasoline by alkylation of reformate. The process involves alkylation using a light olefin feed at low temperature over the zeolite catalyst, MCM-49.
Umansky el al. U.S. Pat. No. 7,476,774 describes a process where light olefins including ethylene and propylene are extracted from refinery off-gases, such as from a catalytic cracking unit, into a light aromatic stream, such as a reformate containing benzene and other single ring aromatic compounds, which is then reacted with the light olefins to form a gasoline boiling range product containing alkylaromatics. The alkylation reaction is carried out in the liquid phase with a catalyst which preferably comprises a member of the MWW family of zeolites, such as MCM-22, using a fixed catalyst bed.
However, in addition to limiting the benzene level in gasoline, current and ongoing regulations restrict the content of residue, which consists of heavy hydrocarbon components with boiling points outside the gasoline boiling range. The US standard specification for automotive spark-ignition engine fuel (ASTM D4814) requires that the residue (heavies) in the gasoline product is no more than 2 volume %. As benzene regulations become more stringent, meeting the heavies level becomes an increasing problem because the light olefins used to alkylate the benzene in the gasoline can undergo undesirable competing reactions, such as olefin oligomerization to produce, for example, C6 to C8 olefins. Subsequent aromatic alkylation reactions result in the formation of heavy components, with boiling points outside of the typical gasoline boiling range. This problem is particularly pronounced with gasoline feeds which contain significant quantities of C6 to C8 olefins since these olefins further contribute towards the production of heavy hydrocarbons boiling above the gasoline range.
According to the present invention, it has now been found that the undesirable formation of heavy components in the alkylation of benzene-containing gasoline streams containing significant quantities of C6 to C8 olefins can be reduced by introducing a once-through alkylation pre-reactor upstream of a main recirculating alkylation reactor.